As is known, in numerous applications, power semiconductor components have to be protected against overcurrent and overtemperature and also, if appropriate, against overvoltage in order to prevent their destruction due to a breakdown. A current sensor, a temperature sensor, if appropriate a voltage sensor and a drive circuit are thus required to realize such a protection arrangement.
FIG. 1 illustrates a conventional protection arrangement. A voltage sensor 1, a temperature sensor 2 and a current sensor 3 are connected to a drive circuit 4, which, for its part, is connected to a power semiconductor component 5 to be protected. The voltage sensor 1 determines the voltage occurring at the power semiconductor component 5, while the temperature sensor 2 ascertains as far as possible the maximum temperature of the power semiconductor component 5 and the current sensor 3 serves for measuring the current sent through the power semiconductor component 5. The output signals of the sensors 1 to 3 are evaluated by the drive circuit 4 in order, in the event of specific limits being exceeded, to effect a turn-off of the power semiconductor component 5 or a power limiting through analog control, so that said component is prevented from being destroyed.
Specifically, the interaction of the three sensors 1 to 3 in terms of the temporal profile is of great importance in order to be able to reliably protect the power semiconductor component 5 during all conceivable thermodynamic processes.
In order to reliably avoid destruction of the power semiconductor component, it is necessary for said power semiconductor component to be operated in the so-called “safe operation area” (SOA) theoretically at any time, that is to say in particular also during fast dynamic processes.
Said SOA will be explained in more detail below with reference to FIG. 2. FIG. 2 illustrates the current i through a power semiconductor component as a function of the voltage U across said component.
In a range a of low voltages such as occur for example between drain and source of a transistor, the SOA is generally limited by the maximum current which the bonding of the housing of the component withstands. At medium voltages in a range b, primarily the temperature prevailing in the power semiconductor component is the limiting factor, the power, that is to say the product of current and voltage, being virtually constant here, so that the thermal resistance in the power semiconductor component is relevant. As the voltage rises further in a range c, it is necessary, finally, for the power to be reduced and ultimately to be brought to zero in order to prevent breakdowns.
The thermal properties have to be taken especially into account in a dynamic consideration of the power semiconductor component. That is to say that the SOA boundary line therefore has to be shifted dynamically to lower values by a protection arrangement as the temperature rises in the region of the power semiconductor component. In other words, as the temperature T rises, the SOA boundary line undergoes transition from a profile A to a profile B and further to a profile C. Correspondingly, the boundaries between the ranges a, b and c may also change slightly.
In practice, problems often arise, then, with regard to reliably detecting and correctly combining the input variables that are essential for the SOA definition, current (range a), temperature (range b), voltage (range c) and —in a dynamic consideration —time. In particular, it is difficult, in the case of high power densities and fast changes in the power loss in a power semiconductor component, to detect the peak temperature thereof, which is often given by the actual chip temperature. Moreover, it is possible only with difficulty to dynamically precisely measure the total current through the power semiconductor component. Furthermore, with the present-day conventional arrangements, it is not possible to monitor at any time the precise power distribution in a power semiconductor component, in particular power transistor, comprising numerous cells in order to avoid instances of local overheating, so-called “hot spots,” which would otherwise lead to direct destruction of the relevant cells.
Current sense devices have hitherto preferably been realized from sense resistors through which flows the total current which is also passed through the power semiconductor component. However, thought has also already been given, for example in the case of a MOS power transistor as power semiconductor component, to designing specific parts, that is to say individual cells, as sense cells and to preferably constructing a current mirror circuit from said cells, which current mirror circuit then determines the current flowing through these sense cells as a measure of the total current. In the case of such solutions, however, it has been shown that the accuracy with which such current sense devices measure the current through a power semiconductor component may be unsatisfactory. Moreover, the temperature behavior is not uncritical here since such individual cells are not reliable for the determination of “hot spots.” This holds true particularly at high voltages and high power densities since the temperature gradients are then large. Modern MOS transistors tend toward the formation of “hot spots” particularly in the limited state.
Similar observations may also be made for voltage sense devices.
Existing protection arrangements have temperature measuring devices for example in the form of additional chips or corresponding sensors applied on the chip of the power semiconductor component. It is thus possible, by way of example, to provide temperature sense transistors in the vicinity of a power transistor as power semiconductor component.
In order, then, to be able to detect a peak temperature as well as possible, a sense transistor should, for example, be moved into a power transistor. An optimum solution here would be one in which each cell of a power transistor is assigned an integrated sense transistor as temperature protection. By correspondingly dividing a power transistor into numerous cells and inserting sensors into these cells, it is thus possible to provide a protection arrangement that operates very well.
In the case of an excessively high rise in the temperature in the cells of a power transistor, care must be taken, moreover, to ensure that the power which is consumed in the power semiconductor component is dynamically regulated back in order thus to prevent the power semiconductor component from being destroyed.
Overall, therefore, a protection arrangement for a power semiconductor component comprising at least two cells has not been disclosed hitherto which is able reliably and with low outlay to establish operation safely in the SOA in order thus to prevent the power semiconductor component from being destroyed by local overheating. This holds true in particular for power semiconductor components used as output stages. Therefore, in order to ensure safe operation, such output stages and generally power semiconductor components are often derated.
For these and other reasons, there is a need for the present invention.